How do I add together two a⨯a images to a single 2a⨯a image in the compositor?
I suggest a different solution:

here's the Compositing setup:



The first node (image node) is an empty image (generate in blender as 100% alpha image) with the full size of your output image (1024x512). When the alpha image is added to the rendered image, the output (file output) is a 1024x512px file.

Add the second scene (again translated in the opposite direction) and you'll have the two renders aligned.